Heretofore, as a toner for the development of an electrophotographic image there has been normally used a particulate toner prepared by a process which comprises dispersing a coloring agent such as dye and pigment, and optionally a magnetic material, a charge controlling agent and in offset inhibitor (lubricant) in a thermoplastic resin in molten admixture, cooling and solidifying the dispersion, finely grinding the solid, and then classifying the grains to obtain a particulate toner having a predetermined grain diameter.
However, the recent trend of image formation is for higher definition. It has been found that this requirement can be effectively met by reducing the grain diameter of the particulate toner and narrowing the distribution of grain diameter.
As the grain diameter of the particulate toner is reduced by the foregoing grinding method, some problems have occurred. Firstly, the present grinding method consumes a huge energy to produce a particulate toner having a reduced grain diameter or narrowed distribution of grain diameter. Further, the produced particulate toner has a broad distribution of grain size and thus tends to include a large amount of unnecessary extremely finely divided powder which essentially requires classification process. However, it is extremely difficult to remove such an extremely finely divided powder by classification, reducing the productivity and hence the yield. The resulting toner is inevitably available at higher cost.
Secondly, it is difficult to homogeneously disperse a coloring agent or other additives in a resin at kneading process. The particulate toner with a reduced grain diameter produced by the grinding process is liable to greater scattering of chargeability due to the maldispersion of a coloring agent, a charge controlling agent, etc., coupled with the reduction of the grain diameter of the toner. This leads to the reduction of definition. Thirdly, the thus-obtained toner does not have uniformity in shape of the grains because it is a product of grinding. This not only causes the reduction of the fluidity of the toner but also produces a finely divided powder when re-ground by agitation upon development. The finely divided powder thus produced causes filming on the carrier and photoreceptor, resulting in the reduction of durability.
For this reason, various polymerization process toners have recently been proposed to eliminate the disadvantages of the grinding process. In particular, the production of toner by suspension polymerization process is advantageous in the control of grain diameter and the reduction of cost and is now under extensive development. However, some problems characteristic of suspension polymerization process have occurred. The suspension polymerization process requires the addition of a dispersant and a stabilizer to the aqueous system to accomplish stable suspension free from agglomeration or destruction of grains. In particular, the addition of a dispersant inhibits the agglomeration or destruction of grains during suspension or polymerization.
However, the addition of a dispersant entails the necessity of rinsing the toner to remove the residual dispersant from the surface of the particulate toner. The residual dispersant on the surface of the particulate toner not only deteriorates the stability of chargeability but also hinders the environmental stability. Further, a large amount of rinsing water or a large amount of rinsing water containing alcohol is required to remove the dispersant. Nevertheless, complete removal of the dispersant is difficult. Moreover, the Use of a dispersant presents a problem of increase of the amount of finely divided grains and production of a large amount of an emulsion. The presence of a large amount of an emulsion or finely divided grains causes the drop in the image density during running, requiring classification treatment.
JP-A-6-33256 (the term "JP-A" as used herein means an unexamined published Japanese Patent Application), which corresponds to U.S. Pat. No. 5,474,870, discloses a process which comprises (1) making a mixed solution collide with each other or with a predetermined plane, (2) immediately passing the mixed solution through an apparatus for throttling the path such that some back pressure and shearing stress acts on the mixed solution, to thereby prepare grains having an average grain diameter of 4 .mu.m to 8 .mu.m, including those having a grain diameter of 3 .mu.m to 12 .mu.m necessary for dry electrophotographic developing toner in a proportion of 13 to 350 % higher than the conventional process, and an extremely narrow distribution of grain size and comprising various additives dispersed therein through less passes than the conventional process, and (3) polymerizing the grains to prepare a toner for developing an electrostatic image that can provide a high definition without any fog and unevenness at a high productivity and a low manufacturing cost.
As mentioned above, when the production of a particulate toner for the development of an electrostatic image is effected in a dispersant-containing system even by the suspension polymerization process proposed in JP-A-6-33256, some problems such as widening of the distribution of grain size and deterioration of stability of chargeability and environmental stability of the toner are left unresolved.